1. Field of the Invention
Apparatus and method for controlling the rate of ingress of radon and other contaminants from the soil into building interiors, thereby reducing exposure of individuals to radon and other soil gas borne contaminants.
2. The Prior Art
Indoor radon has been identified by the United States Environmental Protection Agency (EPA) and the by the United States Surgeon General as the second leading cause of lung cancer in the United States (second only to tobacco smoke). Recent studies have also linked radon with cancers in several other organs of the body, including leukemia in children. An individual's greatest exposure to radon occurs in the indoor environment. Numerous studies have documented that the primary source of indoor radon is radium in the soil. Radon produced by radioactive decay of the radium migrates with the soil gas and enters the building.
One prior approach to controlling entry of radon into buildings is active subslab ventilation (SSV), as disclosed, for example, in U.S. Pat. No. 4,620,398. These systems consist of one or more fans (or similar devices) connected through pipes to the subslab region of a building foundation. Commonly, the connection is formed by drilling holes in the concrete slab floor or subfloor. The fan is oriented to either pressurize or depressurize the subslab region relative to the adjacent interior space (basement or living space). The most common type SSV depressurizes the subslab region relative to the interior space to insure that if air is exchanged between the two spaces the flow will be in the direction from the interior to the subslab space. This direction of flow insures that radon does not enter as a result of the air movement. Applications in which the subslab region is pressurized attempt to force the radon in the vicinity of the foundation or slab away from the building. In practice, this latter technique works primarily by diluting the radon concentration in the soil gas by the introduction of fresh air under the slab. One or the other of these approaches proves to be effective in preventing radon entry into most buildings. The major disadvantages of SSV methods are: 1) cost, 2) lack of effectiveness in some structures, 3) noise and 4) the mechanical systems require maintenance. The cost of this technique ranges from $800.00 to $5,000.00 for residential installations depending on the structure and soil type. A typical installation cost is $1,500.00 for a single family home. Operating costs to run the fan can also be substantial. This relatively high cost appears to serve as a deterrent to implementation of SSV technology. The technique does not work well when the building is placed on low permeability soil with no aggregate (about 20% fail), or when air communications under the slab is blocked (e.g. interior footings, load beams, or heating ducts).
It has been known for many years that water-saturated soil between two electrodes behaves in a manner similar to an electrolytic cell. See Casagrande, L. (1952), "Electro-osmotic Stabilization of Soils," J. BSCE, 39, 51-83; Casagrande, L. (1983), "Stabilization of Soils by Means of Electro-osmosis, State-of-the Art," J. BSCE, 69(2), 255-302; and Lockhard, N. C. (1982), "Electro-osmotic dewatering of clays, III. Influence of clay type, exchangeable cations, and electrode materials," Colloids Surf., 6, 253-269. When a DC voltage is applied, water will be transported between the electrodes. This electro-osmotic technology has been used to remove water from the soil at construction sites, to stabilize embankments, and to increase loading capacity. This phenomenon is sometimes referred to as electrical drainage. Electro-osmotic effects have also been studied for their potential applications in the cleanup of contaminated soil water. See, Segall, B. A., C. E. O'Bannon and J. A. Matthias (1980) "Electro-osmotic Chemistry and Water Quality, J. Geotech. Engrg. Div.," ASCE, 106(10), 1148-1152; Bruell, C. J., B. A. Segall and M. T. Walsh (1992), "electo-osmotic Removal of gasoline hydrocarbons and TCE from Clay," J. Envir. Engrg., ASCE, 118(1), 68-83; and Segall, B. A. and C. J. Bruell (1992), "Electro-osmotic Contaminant Removal Processes," J. Envir. Engrg., ASCE, 118(1), 84-100.
U.S. Pat. No. 1,175,970 discloses an electro-osmotic system for pulling water toward a crop or plant. As described at page 1, lines 69-80, one electrode would be a series of parallel electrodes or metallic wires and the other electrode would be a series of conducting plates. The latter electrode would be placed at a location "of a maximum water content, or even in a suitable spot at a considerable distance away."
U.S. Pat. No. 4,145,270 discloses an electro-osmotic system in which the foundation wall itself is the positive electrode. As is typical of such art, the system is designed to draw water away from the foundation.
U.S. Pat. No. 4,600,486 discloses electro-osmotic apparatus installed on a foundation wall with a positive grid and a negative grid arranged in a coplanar configuration, with the positive grid being located on the foundation wall, above the negative grid also located on the foundation wall.
U.S. Pat. No. 3,070,528 discloses a damp-proofing arrangement wherein positive and negative grids are arranged on opposite sides of a structure.